Electrostatically deformable materials

ABSTRACT

There is disclosed a process whereby non-deformable thermoplastic materials can be made to deform by the introduction of polar groups into these materials.

United States Patent Ciccarelli et al.

ELECTROSTATICALLY DEFORMABLE MATERIALS Inventors: Roger N. Ciccarelli; Dale R. Ims,

both of Rochester, NY.

Assignee: Xerox Corporation, Stamford,

Conn.

Filed: Jan. 22, 1973 Appl. No.: 325,488

Related U.S. Application Data Continuation of Ser. No. 133,732, April 13, 1971.

U.S. Cl 96/l.1; 96/15 Int. Cl G03g 13/00 Field of Search 96/1.1, 1.5; 340/173 TP 3,892,567 July 1, 1975 Primary Examiner-Norman G. Torchin Assistant Examiner-John L. Goodrow [5 7] ABSTRACT There is disclosed a process whereby non-deformable thermoplastic materials can be made to deform by the introduction of polar groups into these materials.

10 Claims, No Drawings ELECTROSTATICALLY DEFORMABLE MATERIALS This is a continuation of application Ser. No. 133,732 filed Apr. 13, 1971.

BACKGROUND OF THE INVENTION This invention relates to xerography and more particularly to novel electrostatic means of forming visible patterns on surface deformable materials.

It is known that variations of charge on a plastic film on a conductive substrate, followed by generally softening it, causes either of two types of surface deformation depending on the methods employed; the first method is known as relief imaging and the second method is known as frost imaging. In frost imaging solid area coverage of the images is provided, whereas in relief imaging only line copy image reproduction may be achieved. In relief imaging charges placed and held on the surface of an insulating film layer on a conductive support experience a force of attraction toward the charges of opposite polarity induced in the conductive substrate. When charge is dissipated or added or when a combination of both operations is performed on the insulating film in a non-uniform fashion by a suitable method, sharp charge differences will be present on said film. These sharp charge differences result in an imbalance of force on the surface of the film causing it to deform if a cold flow material is used. Heat or a suitable solvent may be required for other materials to allow the deformation to take place. A photoconductive film can be made to translate exposure differences 7 into charge differences thereby creating sharp differences in charge density between adjacent areas on the plastic insulating layer resulting in viscous flow at the boundaries. The response in this method is related to differences in charge density of adjacent areas rather than absolute charge densities as in the frost imaging method so that relief imaging is not suited to continuous tone reproduction but results in only outline or line copy images.

In frost imaging, however, a form of surface deformation distinctly different from relief can be induced by charging a thin insulating film resulting in a diffusely reflecting or light scattering surface, the charged surface taking on a frosted appearance. The frost process is discussed in detail in a publication entitled A Cyclic Xerographic Method Based on Frost Deformation by R. W. Gundlach and C. J. Claus, Journal of Photographic Science and Engineering, February edition, 1963 and in US. Pat. No. 3,196,011. The relief imaging process has been described in US. Pat. Nos. 3,055,006; 3,063,872 and 3,113,179.

As mentioned above the frost method may involve the electrophotographic process whereby a latent electrostatic image is produced on a thin dielectric thermoplastic film. This film is then deformed either concurrently with or subsequent to charging by heating or exposure to an atmosphere of solvent vapors which produce a solid area of visible image. By employing the proper sequence of charging and exposure on a plastic overcoated layer, a charge pattern can be created which controls selected wrinkling or frosting of the deformable layer to form solid area images. After the image is made visible by frosting, it may then be frozen by allowing the frosted film to harden by various methods such as by removing the source of heat, solvent vapor or the like used to soften the deformable layer. If it is desired to reuse the same film, the image can be erased after use by simply restoring and maintaining a low viscosity for a sufficient period of time using the very same methods employed to initially soften the film.

While many materials which are normally solid, electrically insulating and are capable of being temporarily softened by the application of heat, solvent vapors or the like, may deform to form relief images by the relief imaging method described above, it is known that these same materials in many instances will not form frost images. In some cases it may be desirable to only have edge or outline imaging as is obtained by relief, however, in many instances it is more desirable to have images of solid area coverage as is obtainable by the frost method. Furthermore, it has also been noted that some thicker surface deformable electrically insulating materials, although being both reliefable and frostable, form comparably poor frost and relief images under the circumstances. In addition, those materials found to be suitable for forming relief images have also, in a number of cases, been found to produce poor results. Certain requirements are imposed on those materials that may be utilized in the deformation process. These include but are not limited to the dielectric strength of the insulating film, conductivity, charge retention, maximum film thickness, viscosity, the ratio of charge retention to viscosity and the proper smoothness of the film surface of the material. Because of these considerations, the selection of compositions for use in either or both frost and relief is rather restricted. Certain thermoplastic materials, for example, that may have the desirable physical properties for commercial relief processes have been found unsuitable for the frost procedure. In addition, many compositions, although apparently possessing all the desirable physical and electrical properties necessary for commercial utility as deformable recording media, have demonstrated that the resulting frost or relief images in a number of instances are poorer in quality, and considerably less suitable than others for commercial application. Thus, by necessity the choice of surface deformable materials for either process above disclosed have thus been limited to a comparatively small number.

In general, it has been found that though there is a vast variety of plastic materials which have the proper physical properties, very few of these materials have the desired electrical properties, i.e. charge retention ability, so that they may be employed in the deformation imaging process. There is, therefore, a demonstrated need for supplying material which may be useful in the deformation imaging process or in providing plastic materials with the proper functionality so that they may be useful in the deformation imaging process.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a surface deformable imaging system which will overcome the above noted disadvantages.

Another object of this invention is to increase the selection of materials easily adapted for use as a surface deformable recording media.

Yet another object of this invention is to provide a method of incorporating the desired functionality into certain plastic materials.

Again, still a further object of this invention is to provide a process which will permit the use of a substantially wider selection of relatively thin materials and films for deformation imaging processes.

Still another object of this invention is to provide thermoplastic materials which deform in the sequential mode without development. a

A further object of this invention is to provide deformable materials having a wider range of varied physical and chemical properties.

Again still another object of this invention is to provide a novel surface deformable composition adapted for use as an image deformable recording medium.

Yet still another object of this invention is to provide a novel surface deformable imaging composition.

These objects and others are accomplished in accordance with the process of the present invention generally speaking by blending polar additives into non-polar (carbon-hydrogen) thermoplastics, and by synthesizing thermoplastics with the proper type and amount of polar group. Polar group materials, that is materials having polar groups or polar additives are materials which exhibit dipole moments more fully described in Physical Chemistry, Farrington Daniels, Robert A. Alberty, and John Wileys Sons Inc., 1955. For example, a thermoplastic material hitherto unusable in the deformation imaging process by virtue of its inability to retain electrostatic ccharge deposited thereon, such as, alphamethylstryene, is supplied with the charge retention properties required for use in the deformation imaging process by incorporating therein compounds having polar groups, such as, carboxylic acid.

Alphamethylstyrene may be anionically polymerized followed by carbon dioxide in one reaction and methanol termination in a second reaction to form the polar carboxylic acid terminated and the non-polar hydrogen terminated polymers respectively. It is found that while the polar thermoplastic deforms, the non-polar material does not. In general, the basic problem encountered with these particular dipole groups isthat deformation requires high fields, usually in the order of about 100 volts per micron for films of about 2 microns thick. The requirement for high fields is overcome by the further discovery that metal salts attached to a polymer substantially increase their performance as deformation imaging materials.

A preferred embodiment of the present invention is accomplished by polymerizing alphamethylstyrene with butyl-lithium to a molecular weight of about 2000 followed by carbon dioxide termination to form the CO Li terminated polymer. When employed in a deformation imaging process, the deformation onset for this polymer occurs when it is charged at room temperature to about a field of positive or negative 45 volts per micron and then heated to about 175C. If other suitable polymers are used then deformation temperatures less than 175C can be obtained, for example, using styrene instead of a-methyl styrene, a deformation temperature of about 100C results. The high softening point of these polymers coupled with their deformation properties signifies that these polar structures retain charge exceptionally well.

In another embodiment of the process of the'present invention styrene is copolymerized with methacrylic acid at about mole percent and then converted to either the lithium or sodium salts respectively having a mean average molecular weight (M) of 4338. When elther material is blended with about 99 weight percent Piccotex 100, a copolymer of vinyltoluene and alphamethylstyrene, the blends containing the lithium and sodium salts deform strongly while those of the acid copolymer compound deform to a lesser extent.

In addition to the differences in electrical properties produced by the incorporation of these polar groups, it is found that the incorporation of these polar groups is only effective to produce deformation imaging materials in a specified concentration range of polar groups to plastic materials.

Although a quanitative approach has shown difficulty in application it has been found in the above lithium compounds that optimum electrical properties have generally been incorporated into this plastic material when about 100 ppm of Li to about 6,000 ppm of Li is incorporated therein. When from about 6,000 to about 10,000 ppm of Li is incorporated therein satisfactory properties are observed, and at about 10,000 ppm or more poor electrical properties are observed.

When calculating ppm for materials other than lithium, the proper correction factors must be applied based on the respective molecular weights of the polar function. So it has been determined that for any plastic material which is comprised of identifiable monomeric units an addition of from about one polar group per 1,000 monomeric units to about one polar group per 5 monomeric units is found to be the range of polar group concentration below and above which unsatisfactory results are obtained. Where the plastic material is not comprised of identifiable monomeric units the range of polar group concentration may be expressed in terms of mole percent so that the' addition of about 0.1 mole to about 20 mole of polar groups defines the above range. Still another method used, relates to expressing additive groups in terms of groups per backbone carbon which results in the above defined ranges being expressed as 1 polar group'per 2,000 backbone carbon atoms to l polar group per 10b'ackbone carbon atomsfor polymers made from ethylenically unsaturated monomers.

The surface deformable material thus obtained may then be employed as hereinbefore described utilizing these techniques more fully described in US. Pat. No. 3,196,011.

Any suitable plastic material may be employed in the process of the present invention. Typical plastic materials include silicone oligomers and polymers; aromatic oligomers and polymers including poly-styrene, vinyltoluene, substituted vinyltoluenes, alphamethylstyrene, halogen derivatives thereof and copolymers and mixtures thereof; polyacrylic acids, methacrylic acids, esters thereof and copolymers thereof; aliphatic amorphous hydrocarbons including polybutenes, polyisobuylenes and highly branched aliphatics having molecular weights 5000; phenoxy oligomers and polymers of poly ortho, meta, and para-phenoxy compounds; triazine and cyanuric acid derivatives.

Any suitable compound having polar groups may be employed in the process of the present invention. Typical compounds having polar groups include halides, esters, ethers, epoxys, quaternary amines; hydroxyl, phenolic, sulfonic acid, carboxylic acid, and the metal salts of these, such as, Li, Na, K, Mg, Ca, Ti, Cr, Fe, Co, Ni, Cu, Ag, Zn, Cd, Al, Sn, As, Se, Te;.alcohols, organic acids and the metal salts thereof, carbon chlorine compounds, carbonyl compounds, hydroperoxides, and

carboxylic esters. Of these the metallic salts are preferred, for example, the lithium and sodium salts of carboxyl containing polymers and copolymers. Polymers having incorporated therein copper and tin salts have also shown satisfactory results.

Any suitable method of charging may be employed in practicing the process of the present invention. Typical methods of charging include electron gun charging in a vacuum, corona charging, friction charging and induction charging more fully described in U.S. Pat. Nos. 2,934,649 and 2,833,930 respectively and roller charging more fully described in U.S. Pat. No. 2,934,650.

In order to form an image, the thermoplastics may be combined with a photoconductor charged and exposed employing any suitable method of exposure. Typical methods of exposure include reflex, contact holographic techniques, non-lens slit scanning systems, and optical projection systems involving lens imaging of opaque-reflection subjects as well as transparent film.

Any suitable method of rendering the image deformable surface visible may be employed in the process of the present invention. Typical of such methods include softening the thermoplastic film with heat and vapor.

Any suitable method of permanently fixing the visible image thus obtained in the deformation imaging process may be employed in the process of the present invention. Typical of such methods include read out and freezing techniques in addition to allowing the deformable surface to cool at room temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To further define the specifics of the invention the following examples are intended to illustrate and not limit the particulars of the present process. Parts and percentages are by weight unless otherwise indicated.

EXAMPLE I In a first reaction about 0.1 mole of cyanuric chloride is dissolved in about 125 ml. of Baker Reagent acetone. About 0.3 moles of m-(m-phenoxy phenoxy) phenol is dissolved in an aqueous sodium hydroxide solution comprising about 125 ml. of deionized water and 12 grams of sodium hydroxide under an inert gas. The sodium m-(m-phenoxy phenoxy) phenate is then added to the cyanuric chloride acetone solution at about l0C dropwise over a period of about 1 hour. The mixture so obtained is stirred for about one hour at about l0C and then stirred for about 2 hours at room temperature and finally for about 4 hours at reflux (57C). About 300 ml. toluene is added and the water from the mixture is decanted. The toluene is washed with about 200 ml. of about a 2% sodium carbonate solution, twice with deionized water, 200 ml. of about a 5% hydrochloric acid solution, and finally twice with deionized water. The toluene solution is dried overnight over a molecular sieve 4A. The toluene is removed in a flash evaporator while heating to a temperature of about 150C at about 4 millimeters (mm) mercury (l-Ig.) obtaining a yield of about 91% of the first reaction product which is 1,3,5-tri (m-phenoxy phenoxy phenyl) cyanurate having a molecular weight of about 922 and substantially the following analysis: C- 74.93%, I-l-4.38%, N-4.57%, O-l6.00% and Na about 36 ppm.

In a second reaction about 0.1 mole of cyanuric chloride is dissolved in about 125 ml. of Baker Reagent acetone. About 0.2 moles of m-( m-phenoxy phenoxy) phenol is dissolved in an aqueous sodium hydroxide solution comprising about 125 ml. water and 16 grams sodium hydroxide reagent grade under an inert gas. The sodium m-(m-phenoxy phenoxy) phenate is added to the cyanuric chloride-acetone solution at about l0C dropwise over a period of about one hour. The mixture is stirred for about one hour at about l0C, then about 4 grams of sodium hydroxide in about 50 ml. of water is added. The mixture is then stirred for about two hours at room temperature, and finally for about four hours at reflux (57C). About 300 ml. of toluene is added and the water removed by decantation. The toluene solution is washed twice with water. The toluene solution is dried overnight over a molecular sieve 4A and finally the toluene is removed by flash evaporation. The second reaction product so obtained is found to have a molecular weight of about 946 and substantially the following analysis: C-73.6%, I-I-4.43%, N- 5.l0%, O-l4.82%, and Na-1.77%. A blend is made of weight percent of the first reaction product with 25 weight of the second reaction product. The themeplastic material thus obtained is solution cast into a deformable imaging layer of about 2 microns thick over a photoconductive surface. The layer is then charged to about volts across the thermoplastic utilizing a corotron, imagewise exposed, recharged, and then heat softened to produce an excellent frost image utilizing those techniques more fully described in U.S. Pat. No. 3,196,011.

EXAMPLE II Under a vacuum of about 10 mm Hg. about 0.085 mole of alphamethylstyrene and about 100 ml. of tetrahydrofuran are distilled into a reaction vessel. This mixture is cooled to about 78C in a dry ice acetone bath. Initiation is accomplished with about 3.1 ml. of nbutyllithium (about 1.6M in n-hexane). The reaction mixture is removed from the bath and allowed to warm to room temperature until a deep red color appears. It is then cooled in a dry ice acetone bath and stirred overnight. Tennination of the reaction is accomplished by rapidly bubbling dry carbon dioxide through the mixture with stirring. The polymer is isolated by desolvation under vacuum. It is then redissolved in toluene and precipitated from a large excess of spectral grade methanol. After filtration and drying a yield of about 5 grams of the polymer is obtained. This material is found to have a molecular weight of 2078 and substantially the following analysis: C-88.92%, H-8.54%, O-l.6l%, and Li-2750 ppm. The material so produced is solution cast into a deformable imaging layer over a photoconductive substrate. The layer is then charged to about 50 volts per micron across the thermoplastic utilizing a corotron, imagewise exposed, recharged, and then heat softened to produce an excellent frost image utilizing those techniques more fully described in U.S. Pat. No. 3,196,011.

EXAMPLE In About 600 grams of diphenyl ether is heated to about 200C to which is added a solution of about 1.77 moles of styrene and about 0.194 moles of methacrylic acid dropwise over a period of about 2 hours. After heating for an additional hour the mixture is cooled to room temperature then added dropwise to a large excess of methanol. After filtration and drying a yield of about 130 grams of polymer is obtained having a molecular weight of about 4338 and the following analysis is obtained: 089.20%, H-7.69%, O-3.17%, and an acid member (mg KOH/gm) 54.43. One weight percent of the prepared polymer is blended with about 99 weight percent of a copolymer of styrene-vinyltoluene. When the mixture is cast into a film the charge retention ability is found to be better than the styrene-vinyltoluene alone, particularly at elevated temperatures. The material so obtained is solution cast into a deformable image layer. The layer is then charged to about 50 volts per micron across the thermoplastic utilizing a corotron, imagewise exposed, recharged and then heat softened to produce an excellent frost image utilizing those techniques more fully described in 1.1.8. Pat. No. 3,196,011.

EXAMPLE IV The copolymer as produced in Example III is reacted in toluene with about a 10 mole excess of sodium methoxide. The polymer salt is isolated by precipitation from isopropyl alcohol thus obtaining a plastic material having a molecular weight of about 4800 and the following analysis: C-85.6%, i-7.59%, and Na-2.48%. One weight percent of the prepared polymer is blended with about 99 weight of a copolymer of styrenevinyltoluene. The product so obtained is solution cast into a deformable imaging layer. The layer is then charged to about 50 volts per micron across the thermoplastic utilizing a corotron, imagewise exposed, recharged and then heat softened to produce an excellent frost image utilizing those techniques more fully described in US. Pat. No. 3,196,011.

EXAMPLE V The acid copolymer as produced in Example III is reacted in toluene with about a 10 mole percent excess of lithium hydroxide monohydrate. The reaction is driven to completion by azeotroping off the theoretical quantity of water. The polymer salt is then isolated by desolvation under vacuum, thus producing a plastic material having the following analysis: C-87.68%, H- 7.72%, O-3.42%, and lei-0.67%. One weight percent of the prepared polymer is blended with about 99 weight of a copolymer of styrene-vinyltoluene. The material so produced is solution cast into a deformable imaging layer. The layer is then charged to about 50 volts per micron across the thermoplastic utilizing a corotron, imagewise exposed, recharged and then heat softened to produce an excellent frost image utilizing those techniques more fully described in US. Pat. No. 3,196,011.

EXAMPLE VI A copolymer of styrene-vinyltoluene is dissolved in cyclohexane and subjected to ozonolysis by bubbling ozone through the solution until a film of the polymer exhibits strong carbonyl and hydroxyl absorptions under infrared spectroscopy. The material so produced is solution cast into a deformable imaging layer. The layer is then charged to about 50 volts per micron across the thermoplastic utilizing a corotron, imagewise exposed, recharged and then heat softened to produce an excellent frost image utilizing those techniques more fully described in US. Pat. No. 3,196,011.

EXAMPLE VI] A film is cast from a mixture of 10 weight percent polyvinyl chloride and 90% styrene-vinyltoluenecopolymer in methylethylketone on an aluminum substrate. The material so produced is solution cast into a deformable imaging layer. The layer is then charged to about 50 volts per micron across the thermoplastic utilizing a corotron, imagewise exposed, recharged and then heat softened to produce an excellent frost image utilizing those techniques more fully described in U.S. Pat. No. 3,196,011.

Although the present examples are specific in terms of conditions and materials used, any of the above listed typical materials may be substituted when suitable in the above examples with similar results. In addition to the steps used to carry out the process of the present invention, other steps and modifications may be used, if desirable. In addition, other materials may be incorporated in the system of the present invention which will enhance, synergize or otherwise desirably affect the properties of the systems for their present use.

Anyone skilled in the art will have other modifications occur to him based on the teachings of the present invention. These modifications are intended to be encompassed within the scope of this invention.

We claim:

1. [n a method of surface deformation imaging comprising providing a thermoplastic material overlying a conductive substrate, imagewise exposing said material, and softening said material thereby allowing said material to deform imagewise under the influence of an applied electrical field, the improvement which comprises employing as a thermoplastic material a blend comprising a non-polar thermoplastic polymer and a thermoplastic polymer containing polar groups chemically associated therewith, the amount of said polymer containing polar groups blended with said non-polar polymer being such that the thermoplastic material contains from about 0.1 to about 20 mole percent polar groups.

2. The method of claim 1 wherein said polar groups are selected from the group consisting of hydroxyl, sulfonic acid, carboxylic acid, organic acid, and metal salts of these; carboxylic acid ester, halide, quaternary amine, carbonyl, hydroperoxide, and epoxy.

3. The method of claim 2 wherein the non-polar polymer comprises a polymerized vinyl aromatic compound.

4. The method of claim 1 wherein the polar groups comprise the lithium or sodium salt of a carboxylic acid group.

5. In a method of surface deformation imaging comprising providing a thermoplastic material overlying a conductive substrate, imagewise exposing said material, and softening said material thereby allowing said material to deform imagewise under the influence of an applied electrical field, the improvement which comprises employing as a thermoplastic material a blend comprising a non-polar thermoplastic polymer and a thermoplastic polymer containing polar groups comprising the lithium salt of a carboxylic acid chemically associated therewith, the amount of said polymer containing polar groups blended with said non-polar polymer being such that the thermoplastic material contains from about to 10,000 ppm lithium.

6. The method of claim 3 wherein the polar polymer comprises a copolymer of a vinyl aromatic compound and acrylic or methacrylic acid or esters thereof.

7. An image deformable recording medium comprising a non-polar thermoplastic material having a polar group material comprising the lithium salt of an organic acid incorporated therein in an amount of from about 0.1 to about 20 mole percent and a conductive substrate underlying said material.

8. The medium as defined in claim 7 wherein from about 100 to about 10,000 ppm based upon said thermoplastic material of lithium in the polar group material is incorporated into said thermoplastic material.

9. The medium of claim 7 wherein said non-polar thermoplastic material is selected from at least one phenoxy polymers; and polymers of triazine or cyanuric acid.

10. The method of claim 2 wherein said polymer containing polar groups is selected from the group consisting of polymers containing metal salts of carboxyl groups. 

1. In a method of surface deformation imaging comprising providing a thermoplastic material overlying a conductive substrate, imagewise exposing said material, and softening said material thereby allowing said material to deform imagewise under the influence of an applied electrical field, the improvement which comprises employing as a thermoplastic material a blend comprising a non-polar thermoplastic polymer and a thermoplastic polymer containing polar groups chemically associated therewith, the amount of said polymer containing polar groups blended with said non-polar polymer being such that the thermoplastic material contains from about 0.1 to about 20 mole percent polar groups.
 2. The method of claim 1 wherein said polar groups are selected from the group consisting of hydroxyl, sulfonic acid, carboxylic acid, organic acid, and metal salts of these; carboxylic acid ester, halide, quaternary amine, carbonyl, hydroperoxide, and epoxy.
 3. The method of claim 2 wherein the non-polar polymer comprises a polymerized vinyl aromatic compound.
 4. The method of claim 1 wherein the polar groups comprise the lithium or sodium salt of a carboxylic acid group.
 5. IN A METHOD OF SURFACE DEFORMATION IMAGING COMPRISING PROVIDING A THERMOPLASTIC MATERIAL OVERLYING A CONDUCTIVE SUBSTRATE, IMAGEWISE EXPOSING SAID MATERIAL, AND SOFTENING SAID MATERIAL THEREBY ALLOWING SAID MATERIAL TO DEFORM IMAGEWISE UNDER THE INFLUENCE OF AN APPLIED ELECTRICAL FIELD, THE IMPROVEMENT WHICH COMPRISES EMPLOYING AS A THERMOPLASTIC MATERIAL A BLEND COMPRISING A NON-POLAR THERMOPLASTIC POLYMER AND A THERMOPLASTIC POLYMER CONTAINING POLAR GROUPS COMPRISING THE LITHIUM SALT OF A CARBOXYLIC ACID CHEMICALLY ASSOCIATED THEREWITH, THE AMOUNT OF SAID POLYMER CONTAINING POLAR GROUPS BLENDED WITH SAID NON-POLAR POLYMER BEING SUCH THAT THE THERMOPLASTIC MATERIAL CONTAINS FROM ABOUT 100 TO 10,000 PPM LITHIUM.
 6. The method of claim 3 wherein the polar polymer comprises a copolymer of a vinyl aromatic compound and acrylic or methacrylic acid or esters thereof.
 7. An image deformable recording medium comprising a non-polar thermoplastic material having a polar group material comprising the lithium salt of an organic acid incorporated therein in an amount of from about 0.1 to about 20 mole percent and a conductive substrate underlying said material.
 8. The medium as defined in claim 7 wherein from about 100 to about 10,000 ppm based upon said thermoplastic material of lithium in the polar group material is incorporated into said thermoplastic material.
 9. The medium of claim 7 wherein said non-polar thermoplastic material is selected from at least one member of the group consisting of: silicone polymers; vinyl aromatic polymers; polymers and copolymers of acrylic acid, methacrylic acid and esters thereof; polymers comprising aliphatic amorphous hydrocarbons; phenoxy polymers; and polymers of triazine or cyanuric acid.
 10. The method of claim 2 wherein said polymer containing polar groups is selected from the group consisting of polymers containing metal salts of carboxyl groups. 